docs/bpf: Update btf selftests program and add link

Commit c64779e2

("selftests/bpf: Merge most of test_btf into test_progs")
renamed the BTF selftest from 'test_btf.c' to 'prog_tests/btf.c'.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/tencent_1FA6904156E8E599CAE4ABDBE80F22830106@qq.com